#include <iostream>
#include <vector>
using namespace std;
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
 public:
  int maxDepth(TreeNode* root) {
    if (root == nullptr) return 0;
    return max(maxDepth(root->right), maxDepth(root->left)) + 1;
  }
};
